The invention relates to solution polymerization of conjugated dienes and vinyl aromatic compounds. The invention further relates to random copolymers of the conjugated dienes and vinyl aromatic compounds in which more than 60% of the monomer units are arranged in an arbitrary manner.
Many processes for the preparation of substantially random copolymers of conjugated dienes and vinyl aromatic compounds are well known in the art. Typically, the solvents described for the known polymerization processes include essentially any aliphatic, cycloaliphatic or aromatic compound or a mixture of these compounds, provided that the compound is essentially inert. Cyclohexane alone or in combination with another compound is a highly preferred solvent in prior art processes. Cyclopentane can be found among the compounds actually mentioned as suitable solvents although not exemplified.
An obvious reason for selection of cyclohexane is the well known dissolving ability of cyclohexane for the reaction components and the reaction products of polymerization processes of aromatic vinyl compounds and conjugated dienes. Furthermore the use of cyclohexane as solvent is an economical attractive choice because of its ready availability and its relatively low price.
The random copolymerization processes referred to above are suitably carried out at temperatures in the range of from 50.degree. C. to 110.degree. C. Often compounds having a lower boiling point than cyclohexane, i.e. lower than 80.degree. C., are added to the cyclohexane containing polymerization medium, in such concentrations that the solvency power is still sufficient, in order to provide enough vapor pressure to be able to remove the heat of polymerization by means of evaporative cooling.
Isopentane and n-hexane are known to be suitable for lowering the boiling point of cyclohexane.
It is an object of the present invention to replace a major part of the cyclohexane component of the solvent used in prior art processes for the preparation of substantially random copolymers of conjugated dienes and vinyl aromatic compounds, by a more volatile component while maintaining sufficient solvency during all stages of the polymerization. This was thought to be attractive since it was assumed that this would lead to considerable savings in the steam coagulation of the cement, the solvent recovery and the polymer drying operation. Moreover if a lower boiling solvent were used as the sole solvent in the entire polymerization process further savings could be achieved in solvent storage handling and purification.